This invention relates to vertical direct fired strip heating furnaces in continuous annealing furnaces for heating steel strips.
Although it is a well known fact that a flame cleaning process is frequently used in a continuous zinc plating process, in this process, in the initial stage of the heating of the steel strip, in order to decompose and clean the rolling oil adhered to the surface of the steel strip, means for heating the steel strip in a slight oxidation atmosphere is employed, and in general, a furnace having a direct fired combustion heating system for partly burning the fuel is used.
The conventional vertical direct fired strip heating furnace of this kind has been constructed with one heating chamber, and this limits the processing capacity. The realization of a vertical direct fired strip heating furnace constructed with more than two heating chambers has been strongly demanded. Also, recent technical needs for energy savings have resulted in a strong demand for the realization of a vertical direct fired strip heating furnace provided with a plurality of passages which consists of more than two chambers.
In the conventional direct fired strip heating furnace, since it is of the one heating chamber construction, when the processing capacity is increased, due to a limit of the furnace height because of economic reasons, there is a limit in the heating temperature, and the extra load tends to be applied to the succeeding indirect heating reduction chamber. In case the processing capacity is increased too much, the temperature is limited to an extent which prevents the realization of the original process of flame cleaning. This is a big drawback of the conventional furnace. Also, when heating is done in one chamber, the combustion gases are exhausted at the furnace top portion, thus making the thorough utilization of exhaust gases difficult, and a gas sealing device provided at the opening portion for introducing the steel strip to the heating chamber must be constructed to widthstand the high temperature gases.
The technical concept of preheating the steel strip with combustion exhaust gases of the vertical direct fired strip heating furnace (as disclosed in U.S. Pat. No. 3,532,329) is known, but this technique is such that the preheating chamber and heating chamber for treating the steel strip by exhaust gases of the direct fired strip heating chamber are not communicated, namely, the steel strip passing the preheating chamber is exposed once to the atmosphere, and thereafter is introduced to the heating chamber. In this case, in order not to cause excessive oxidation of the surface of the steel strip exposed to the atmosphere, there are problems such as that the preheating temperature of the steel strip must be limited to a low temperature, and also that the gas sealing device at the opening portion for introducing the steel strip to the heating chamber is required to have a construction capable of withstanding the high temperature, similar to the situation in the conventional vertical direct fired strip heating chamber consisting of one chamber.
In the present invention, in order to solve the foregoing problems, more than two heating chambers are provided in communication. Heating chamber as mentioned herein means a direct fired heating chamber or a preheating chamber. With the foregoing arrangement, a sufficient heating temperature is secured to meet with the necessary processing capacity, and moreover the excessive oxidation of the surface of the preheated steel strip is prevented, the exhaust gas temperature from the preheating chamber is lowered to accomplish an energy saving, the thermal requirements for the gas sealing device at the opening portion for introducing the steel strip is relieved, and, an important thing to be noted here, the furnace is provided with a protection countermeasure for the rolls.
In general, the direct fired strip heating furnace whose primary purpose is to clean the surface of the steel strip by flame performs the flame cleaning process effectively, and also for the purpose of improving the heating efficiency, it is operated at high a temperature ranging from 1000.degree. C.-1250.degree. C. Accordingly, in order to use metal inside furnace rolls economically at such high temperatures, it is necessary to hold the temperature of the inside furnace roll chamber below at least 1000.degree. C. Moreover, in the low temperature region where the temperature of the steel strip is not sufficiently high, then in order to prevent damage to the rolls due to thermal stress generated on the body of the rolls, adjustment of the temperature in the inside furnace roll chamber to a proper range is required.
When there is a big difference between the atmospheric temperature in the circumference of the rolls and the temperature of the passing steel strip, a big thermal stress occurs in the axial direction of the body of the rolls, and in the worst case, cracks may be caused to occur in the body of the rolls. Namely, the center portion of the body of the rolls contacting the low temperature steel strip is constantly cooled by the steel strip and as a result, there occurs an immensely large temperature difference between such center portion and the shoulder portion not contacting the cooled steel strip. According to actual measurement by the inventors, in case the temperature difference is large and may reach 350.degree. C. to 400.degree. C., and thermal stress sufficient to break down the body of the rolls a short time is generated. Normally, in order to hold the thermal stress generated on the body of the rolls to a degree that produces no actual damage from a practical standpoint, it is necessary to hold the atmospheric temperature in the roll chamber above the temperature of the passing steel strip or within temperatures of the steel strip temperature plus 500.degree. C. In this regard, the reason for making the atmospheric temperature above the temperature of the passing steel strip is not to cool the steel strip in the roll chamber. To protect the rolls, water cooling jackets may be disposed in the circumference of the inside furnace rolls to cool the surfaces of the rolls, or the rolls may be cooled indirectly by disposing an air cooling pipe. However, these methods are accompanied by various problems such as danger of water leakage, dew condensation, only a slight cooling of the atmospheric gases in the circumference of the inside furnace rolls, as well as the fact that the inner surface of the body of the rolls is heated by the radiating gases of high temperature filled in the inner surface of the inside furnace rolls, and in addition to thermal stress in the axial direction, a temperature difference is generated between the inner and outer surfaces of the cylinder of body of the rolls to increase the thermal stress in the radial direction.